Role of Protease Activated Receptor 1 and 2 Signaling in Hypoxia-Induced Angiogenesis

Abstract

Objective— Tissue factor (TF) initiates coagulation and indirectly triggers thrombin-dependent protease activated receptor (PAR) signaling. The TF–VIIa complex also directly cleaves PAR2 and promotes angiogenesis in vitro in TF cytoplasmic domain-deleted (TF δCT ) mice. Here we address the effect of PAR1 and PAR2 deficiency on angiogenesis in vivo. Methods and Results— In hypoxia-driven angiogenesis of oxygen induced retinopathy (OIR), wild-type, PAR1 −/− , PAR2 −/− , and TF δCT mice showed a comparable regression of the superficial vascular plexus during the initial exposure of mice to hyperoxia. However, TF δCT mice revascularized areas of central vaso-obliteration significantly faster than wild-type animals. Pharmacological inhibition of the TF–VIIa complex, but not of Xa, and blockade of tyrosine kinase receptor pathways with Gleevec reversed accelerated angiogenesis of TF δCT mice to revascularization rates observed in wild-type mice. Genetic deletion of PAR2, but not of PAR1, abolished enhanced revascularization of TF δCT mice. PAR1 knock-out animals were indistinguishable from wild-type mice in the model of retinal neoangiogenesis and angiogenesis-dependent subcutaneous tumor growth was unaltered in PAR1- and PAR2-deficient animals. Conclusion— Loss of the TF cytoplasmic domain results in accelerated hypoxia-induced angiogenesis mediated by TF–VIIa signaling. PAR2 signaling is sufficient for this proangiogenic effect without apparent contributions of mouse host cell PAR1.